The invention relates to an intervertebral implant in accordance with the independent patent claim.
Intervertebral implants exist in many different embodiments. As their name indicates, they are implanted between two adjacent vertebrae and are used in particular when the intervertebral disc which is arranged there is not capable of functioning or is highly impaired in its functioning. A possible and relatively frequently chosen kind of treatment for such illnesses is to remove the vertebral disc or parts of it and to grow together (fusion) the two mutually adjacent vertebrae between which the damaged vertebral disc is or was arranged. In order to further the growing together of the vertebrae the implant can be filled with bone shavings or bone substitute material. The fusion of the adjacent vertebrae should take place in such a manner that the vertebrae have their normal distance from one another during and after the growing together as if the intact intervertebral disc were still present, because otherwise the functioning of the spinal column as a whole can be considerably impaired.
The implant serves for attaining the primary stability in the first phase after the implantation and must in this phase take up the stresses which would act on an intact intervertebral disc. After a time duration of typically six to nine months the secondary stability is achieved in that the bone of the two adjacent vertebrae has grown together (fused) through the implant, so that the two adjacent vertebrae are arranged with a normal spacing with respect to one another, as if an intact vertebral disc were still present between them. The two adjacent vertebrae are however firmly connected (fused) to one another and can take up the stresses.
For maintaining the distance of two adjacent vertebrae from one another during the first phase after the implantation, that is, during the phase in which the bones of the adjacent vertebrae have not yet grown together (fused), there are various implants and also different operating techniques which already differ already through the manner of the access to the spinal columnxe2x80x94from the anterior or from the posterior.
Thus for example an intervertebral implant is known from U.S. Pat. No. 5,192,327 which is formed as an oval ring-like element. A plurality of elements of this kind can be put together to form a column, the longitudinal axis of which extends after the implantation approximately in the direction of the spinal column, in order also to be able where appropriate to replace a vertebra. The end sides of this implant are open so that bone can grow in into the implant and a bone fusion can take place. The implant can be filled with bone shavings or bone substitute material for furthering the bone fusion. A plastic material which is reinforced by carbon fibers is proposed for this implant in order to be able to monitor with the help of X-rays the progress of the growing together of the bones of the adjacent vertebrae inside the implant. The titanium which is frequently used for implants of this kind does not permit a monitoring of the progress during the growing together of the bone of the two vertebrae inside the implant, since the implant of titanium is not transparent for X-rays.
The implant which is described in U.S. Pat. No. 5,192,327 is principally suitable for the implantation with an anterior access. Otherwise, it is the case in this implant that the implant can frequently not ensure the required primary stability alone and thus further fasteners or stabilizers respectively which must be connected to the vertebrae (e.g. by means of pedicular screws) become necessary in addition to the implant in order to ensure the required primary stability after the operation and in order not to endanger the success of the operation. If then after several months the secondary stability is achieved (through the growing together of the vertebrae) then the additional fasteners or stabilizers respectively can be removed in a further operation.
In order to avoid the application of additional fasteners or stabilizers respectively, which make the operation more complicated (and also more cost intensive) and which can moreover later be removed by means of a further operation, hollow cylindrical implants (so-called xe2x80x9ccagesxe2x80x9d) have already been proposed, for example in U.S. Pat. No. 5,015,247, which are screwed in between adjacent vertebrae with their longitudinal axis transverse to the direction of the spinal column. These hollow cylindrical implants are provided with an outer thread which cuts itself in with the thread into the two vertebral bones between which the implant is inserted during the screwing in. Through this the implants are secured against a sliding out. The implants are provided in the wall with openings (holes), so that bone can grow through the implant, so that a bone connection (fusion) of the two vertebrae can come about after several months.
These implantsxe2x80x94the cagesxe2x80x94have the advantage that they are the only implants which are required during the operation; that is, no additional fasteners or stabilizers are required. A further operation at a later time point for the removal of additional fasteners or stabilizers respectively is thus not necessary. In addition in the use of implants of this kind the access both from the anterior and from the posterior is relatively uncomplicated from the point of view of the surgical technique.
Previously cages of this kind have been manufactured of coarsely blasted titanium, since on the one hand the correspondingly porous titanium surface furthers the growing in or the growing on respectively of bone, but has above all however the required stability on the one hand; on the other hand however it also has a certain elasticity in order to be able to reliably take up the stress acting on the implant in the first phase of the operation, so that additional fasteners or stabilizers respectively are not required. Disadvantageous is that the progress of the bone fusion inside the cages can practically not be monitored in cages of this kind.
The object of the invention is therefore to propose a hollow cylindrical intervertebral implant of this kind (a xe2x80x9ccagexe2x80x9d) which is on the one hand alone suitable for taking up the stresses acting on the implant in the first phase of the operation, but is at the same time however suitable for enabling a monitoring of the bone fusion through imaging processes (X-rays, MRI, CT, etc.) inside the implant.
This object is satisfied by the intervertebral implant which is characterized by the features of the independent patent claim. Particularly advantageous embodiments result from the features of the subordinate patent claims.
This object is satisfied in particular by an intervertebral implant with a hollow cylindrical shape, said implant being provided with an outer thread for screwing in the implant into the intermediate space between two adjacent vertebrae. The implant is provided with openings in its wall through which bone can grow in into the implant, with the implant being manufactured of a material which is transparent for X-rays.
The implant in accordance with the invention has the advantages that on the one hand it can take up the stresses acting on the implant in the first phase after the operation, that it is the only implant and no additional fasteners or stabilizers are required, that the access is possible in a relatively simple way both from the anterior and from the posterior, and that the progress of the bone fusion inside the implant can be monitored with the help of X-ray investigations or other imaging methods (MRI, CT, etc.).
In an advantageous exemplary embodiment of the implant in accordance with the invention the material which is transparent for X-rays is a plastic and can even be manufactured of a plastic only. This plastic can for example be present in a fiber form and be wound to form the desired shape of the implant and then be subjected to a compression sintering. An implant of this kind has no additional reinforcement fibers, but rather can be manufactured of the plastic fibers themselves. In the compression sintering the basic orientation of the plastic molecules of the fibers is preserved, through which the stiffness is imparted to the implant; the fibers are however melted during the compression sintering to such an extent that they combine with one another and form a firm compound in the shape of the implant.
In an advantageous exemplary embodiment of the implant in accordance with the invention the material which is transparent for X-rays comprises a plastic which is reinforced with fiber material.
In this, different variants come under consideration, as will be explained in detail. Thus for example endless fiber reinforced plastics (xe2x80x9ctapesxe2x80x9d) can be wound and then pressed and where appropriate the thus manufactured body also surrounded by a plastic. With quasi endless fiber reinforced materials (xe2x80x9ccommingled stretch broken yarnxe2x80x9d)xe2x80x94these are materials in which the endless fibers are broken, but commingled with one another in order to impart a certain longitudinal elasticity to the materialxe2x80x94it is likewise possible to wind and then to press and then to surround the thus manufactured body where appropriate with a plastic. In so-called long fiber reinforced plastics (fiber lengths typically in the range from 3 mm up to 6 mm or even 10 mm) the discrete fibers are already present in the plastic and the implant is manufactured e.g. through extrusion. Finally, short fiber reinforced plastics also come under consideration (fiber lengths typically in the range from 0.1 mm to 0.4 mm), which can in particular be processed to form the implant in injection moldable plastics. Further advantageous variants can also result from the combination of the different manufacturing techniques.
The named materials are particularly suitable for taking up the loads which act on the implant immediately after the operation and are naturally transparent for X-rays, so that the progress of the bone fusion inside the implant can be monitored with the help of X-ray investigations.
In a further development of the implant the plastic is an injection moldable plastic. The injection molding technique permits a reliable and at the same time economical manufacture of the implant.
The fiber material advantageously comprises carbon fibers and the plastic is preferably polyether ether ketone (PEEK). The carbon fibers take up the loads arising immediately after the operation in a reliable way and can be injection cast in polyether ether ketone because this is an injection moldable plastic which is also biocompatible in addition.
In an advantageous exemplary embodiment of the implant in accordance with the invention the implant comprises a hollow cylindrical body of fiber material which is embedded in plastic, with the hollow cylindrical body of fiber material extending over the entire length of the implant. This exemplary embodiment is distinguished in that the taking up of the load can take place practically over the entire length of the implant and thus the stress on the implant can be distributed over a relatively large area.
In another exemplary embodiment of the implant the implant comprises three bodies which are manufactured of fiber material, which are formed in ring shape and which are embedded in plastic. The taking up of the load takes place in the region of the bodies which are formed in ring shape and can be distributed differently over the length of the implant.
In a first variant with three such bodies which are formed in ring shape, one of the bodies which are formed in ring shape is arranged in the central region of the implant and the other two in the respective end region of the implant. This allows a stressing of the implant both in the central region and in the end regions.
In a further variant all three ring-shaped bodies are arranged more or less in the central region of the implant. The end regions of the implant are in each case provided with a chamfer. The taking up of the load takes place in this variant more or less exclusively in the central region, for which reason all ring-shaped bodies are arranged in this region. This is the case because the end region is provided with a chamfer and in a first phase after the operation has no contact there with the bone which takes up the load. The taking up of the load should thus intentionally take place in the central region in this exemplary embodiment. The chamfer in the end regions facilitates the rotating in of the implant during the operation.
In a further exemplary embodiment of the implant in accordance with the invention the implant comprises a body of fiber material which winds uniformly helically around the longitudinal axis of the implant and which is embedded in plastic, and indeed in such a manner that the ridges of the outer thread of the implant are arranged around the windings of the helical body and the base of the outer thread is arranged between the windings of the helical body. The ridges of the outer thread are thus so to speak reinforced by the helically winding body of fiber material, through which a particular stability is imparted to the thread. In addition the helically winding body of fiber material also ensures a taking up of the load over the entire length of the implant.
Finally, in all above named exemplary embodiments the surface of the implant can be provided with a layer which furthers the osteo-integration, in particular with a hydroxylapatite ceramic layer.